Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a wireless communications system or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications system.
User equipments may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server.
The wireless communications system covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is a geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP GSM EDGE Radio Access Network (GERAN), a user equipment has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. EDGE is an abbreviation for Enhanced Data rates for GSM Evolution.
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In D2D communications user equipments that are in the proximity of each other may discover one another. This is referred to as D2D device discovery. In D2D device discovery user equipments may receive a beacon signal from another user equipment and discover that it is in the proximity of the another user equipment. This may be performed without any direct interaction by the radio access network.
However, there may very well be an indirect radio access network involvement, such as, e.g. the network may inform a first user equipment how to transmit the beacon signal and/or inform a second user equipment about how the beacon signal is transmitted, which would simplify the detection of the beacon signal by the second user equipment. In this type of network assisted D2D device discovery, the radio access network may e.g. allocate resources for the beacon signal so that transmitting and receiving user equipments know what time and frequency resources are being used for D2D device discovery. In other words, when and at what frequencies the beacon signal should be transmitted and scanned for or received at.
It should be noted that the beacon signal transmitted by the first user equipment is typically not only intended for the second user equipment. Multiple user equipments may detect the beacon signal and thus conclude whether or not they are in the proximity of the first user equipment.
Once the user equipments discover the proximity of each other through D2D discovery, the user equipments and/or the network may initiate the establishment of the D2D link between the two user equipments. This is commonly referred to as D2D bearer establishment and is not described in any further detail hereinafter.
The beacon signal from the user equipments are commonly transmitted in a transmission bursts of a certain power and with a relatively short duration e.g. in the range of microseconds (μs). In order to facilitate proximity detection in D2D device discovery, the transmission bursts are transmitted continually with a certain transmission interval.
FIG. 2 shows an example of a conventional beacon signaling according to the prior art in a wireless communication system 100 as shown in FIG. 1.
It can be shown that the energy EB and transmission interval TB of the transmission bursts of the beacon signal determines the range and latency of the proximity detection in D2D device discovery.
However, since beacon signals from user equipments may be take place in an operator licensed spectrum that is used for ordinary cellular communications, the beacon signals may cause interference for the ordinary cellular communications.